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Dec. 3, 2010 -- The FDA says Align Technology Inc., maker of the popular Invisalign "invisible braces," didn't tell the agency about several life-threatening allergic reactions to the product.

The FDA learned of the allegedly unreported events during a July 2010 inspection of the firm. In its warning letter, the FDA says Align Technology failed to tell the agency how it was going to improve its reporting procedures.

In a news release, Align Technology says it did send the FDA the information it requested, but that "our response of November 8, 2010 may have crossed in the mail with the FDA's warning letter." The FDA letter is dated Nov. 17, 2010.

Align Technology is based in San Jose, Calif. Company CEO Thomas Prescott told the San Jose Mercury News that the Invisalign system has been used by more than 1.3 million patients.

Some of those patients appear to suffer serious allergic reactions to the product. The FDA letter describes two complaints:

On Nov. 2, 2007, a patient reported "swollen, irritated, and sore" lips and gums. Similar incidents previously reported to the FDA required hospitalization of the patient.